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Unformatted text preview: 1 Mar. 20, 2006 Phy107 Lecture 22 1 Course Essay • 500-750 word typed essay due Wed. Apr. 26 • First deadline: Fri. this week (Mar. 24) turn in Topic and Paragraph Description • Topic ideas: – Nobel prize winner: work & importance – Big science project: mission & successes – Bogus or contentious science: any scientific support? – Science topic: achievements and outlook See course web site for details and resources Title / paragraph example Topic: Quantum Computers Paragraph: Over the last decade, scientists have developed new approaches to computing using basic ideas of quantum mechanics. Individual atomic particles are used as ‘bits’ of a computer, but instead of representing only ‘0’ and ‘1’, the quantum-mechanical wavefunction is used to simultaneously represent an infinitely variable range of values. Description of topic Outlook What else I will write Science aspect Mar. 20, 2006 Phy107 Lecture 22 3 From Last Time… • Light shows both particle and wavelike properties • Interference is an example of wavelike props. • Photons are particles of light in the particle-like description. • An Experiments demonstrating this was – Photoelectric effect • Einstein received Nobel prize for his explanation of the photoelectric effect. Mar. 20, 2006 Phy107 Lecture 22 4 Photoelectric effect summary • Light is made up of photons, individual ‘particles’, each with energy hf • One photon collides with one electron- knocks it out of metal. • If photon doesn’t have enough energy, cannot knock electron out. • Intensity ( = # photons / sec) doesn’t change this. Minimum frequency (maximum wavelength) required to eject electron = hc " = 1240 eV # nm " Mar. 20, 2006 Phy107 Lecture 22 5 Photoelectric effect question An electron is bound inside copper by a ‘binding energy’ of 4 eV. Which wavelength will eject electrons from copper? A. 300 nm B. 500 nm C. 700 nm Minimum photon energy to eject electron is 4 eV. Corresponding photon energy is given by So λ max =310 nm 4.0 eV = hc " = 1240 eV # nm " nm Mar. 20, 2006 Phy107 Lecture 22 6 Compton scattering • Collision of photon and electron in vacuum • Photon loses energy, transfers it to electron • Photon loses momentum transfers it to electron • Total energy and momentum conserved Before collision After collision Photon energy E = hf Photon mass = 0 Photon momentum p = E / c 2 Mar. 20, 2006 Phy107 Lecture 22 7 Compton scattering • Photons can transfer energy to beam of electrons. • Determined by conservation of momentum, energy. • Compton awarded 1927 Nobel prize for showing that this occurs just as two balls colliding. Arthur Compton, Jan 13, 1936 Mar. 20, 2006 Phy107 Lecture 22 8 Compton scattering question A green photon collides with a stationary electron. After the the collision the photon color is A. unchanged B. shifted toward red C. shifted toward blue Photon transfers energy to electron....
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This note was uploaded on 01/05/2012 for the course PHYS 107 taught by Professor N/a during the Spring '08 term at Wisconsin.
- Spring '08